Millions of sap-sucking insects are to be introduced to control Japanese
knotweed. But what kind of insect is the new gardener's friend?

The psyllid is the name given to a whole family of jumping plant lice that are found all over the world. But the different species tend to be very "host specific" meaning they will only eat one type of plant.

This makes them very useful in biocontrol because they can be released to kill one type of plant without affecting other wildlife.

The psyllid Aphalara itadori, to be released in Britain, only eats Japanese knotweed. The insects were brought to the UK from a specific location in southern Japan where the plant grows on rocky volcanic slopes and the climate is similar to the UK.

The aphid-like insect is about 2mm long and is an orange or brown colour. Typically the psyllid will only live for a few weeks and go through two or three life cycles in one summer. It lays thousands of eggs which hatch and immediately start sucking sap from the plant through its sharp mouth.

The winged insects are currently being bred in captivity in Britain but could spread once they are released, using the wind to carry them further.

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However psyllid will be eaten by native predators like spiders and wasps and should die out once Japanese knotweed is brought under control.

Cabi, the not-for-profit organisation that is rolling out the introduction of the psyllid, put eggs on around 90 other plants like the knotweed but none were able to develop as adults.

They concluded that this made the species the "perfect candidate" for the first release of a non-native species to control another pest.

Plantlife, the plant charity, has suggested that three additional plants (Northern Knotgrass, Tasteless Water-pepper and Small Water-pepper should also be tested prior to release amid concerns the psyllid may affect these species.